1. Field of the Invention
This invention relates to a power steering apparatus having an electric motor as a source to generate a steering assisting power and, in particular, to a power steering apparatus provided with an electromagnetic clutch in a power transmission system exetending from the electric motor to a steering mechanism.
2. Description of Related Art
Recently, a power steering apparatus employing an electric motor as a source to generate a steering assisting power has been in practical use. The apparatus is intended to reduce a burden of labor required for operating a steering wheel by assisting the performance of a steering mechanism with the rotative power of a steering assisting electric motor disposed in the steering mechanism and driven on the basis of results of detecting steering torque.
The power steering apparatus, as compared with a hydraulic power steering apparatus employing a hydraulic actuator as a source to generate a steering assisting power, has an advantage that appropriate characteristics corresponding to vehicle speed, frequency of steering operation, and running condition can be exhibited because the characteristics can be easily changed with the rotation control of the electric motor. However, this apparatus is followed by such a problem as difficulty in obtaining an electric motor of small size which is capable of generating sufficient rotative power for assisting steering. Further problem is that inertia of the rotating parts of the electric motor influences the steering operation. Hence, such means as disposing an appropriate speed reduction unit between an output shaft of the electric motor and a transmission shaft connected to the steering mechanism has hitherto been employed so as to solve the abovesaid first problem by the power increasing action of the speed reduction unit. For solving the second problem, provided is a structure that an electromagnetic clutch is arranged between the output shaft of the electric motor and the transmission shaft connected to the steering mechanism so that both shafts may be engaged with each other, only when required, by the action of the electromagnetic clutch.
The electromagnetic clutch is provided with friction plates respectively rotating in accordance with the output shaft and transmission shaft, a driving coil fixedly disposed on the back of the friction plate on the side of the output shaft, and an armature part disposed on the back of the friction plate on the side of the transmission shaft. On the other hand, the action of the armature part following excitation of the driving coil presses both friction plates to each other and also engages both shafts with each other, and, on the other hand, the action of the armature part following de-excitation of the driving coil releases mutual pressing of friction plates and disengages both shafts from each other. Until today, there has been such a structure that an output shaft on which a friction plate on one side and a retainer therefor are coaxially fitted and a transmission shaft on which an armature part formed integral with a friction plate on the other side is coaxially fitted are separately supported on the same shaft, and respective supported ends of both shafts are disposed oppositely to each other at an appropriate distance. As the reduction unit, a planetary gear reduction unit or planetary roller reduction unit, which can coaxially be composed of the output shaft of the motor and transmission shaft connected to the steering mechanism and is easily reduced in size, is used and disposed on one side of the electromagnetic clutch side by side.
In the conventional steering apparatus thus composed, an extremely high precision of working is required for supported parts of both shafts as well as a fitting part between both shafts and the retainer or armature part in order to obtain a fixed initial gap between both friction plates of the electromagnetic clutch. Both friction plates supported by the cantilevered armature part and retainer can't be prevented from tilting and alike suffer such a problem as requiring a complicated gap adjusting operation and a long period of time for assembling to obtain a uniform initial gap between them. Further, even if care is fully taken for working and assembling, complete prevention of insufficiency in setting the initial gap is substantially impossible. If the initial gap is too large, slip occurs between both friction plates when engaging each other and power transmission is hampered whereas, if the gap is too small, offensive abnormal noise is generated by slide contact of both friction plates when disengaged, thereby insufficiency in performance being caused.
When a reduction unit as described above is provided between the output shaft of the motor and transmission shaft extending to the steering mechanism, lubricant (grease) applied to rotating parts scatters to the side of the electromagnetic clutch and sticks to its friction plates upon operation of the reduction unit and the engaging action is unfavorably hampered.